Bed RestPremature BirthInfant, PrematureInfant, NewbornCervix UteriPregnancyObstetric Labor, PrematureBirth WeightHead-Down TiltInfant, Low Birth WeightWeightlessness SimulationGestational AgePregnancy OutcomeWeightlessness CountermeasuresRestCongenital AbnormalitiesAbortion, VeterinaryPregnancy ComplicationsInfant, Premature, DiseasesCardiovascular DeconditioningBronchopulmonary DysplasiaSpace FlightFetal DeathBedsAbortion, SpontaneousInfant MortalityInfant, Small for Gestational AgePregnancy Complications, InfectiousUterine Cervical NeoplasmsWeightlessnessRespiratory Distress Syndrome, NewbornAnimals, NewbornRisk FactorsOrthostatic IntoleranceBirth RateCervical RipeningLower Body Negative PressureRetrospective StudiesUterine Cervical IncompetenceBirth OrderBirth CertificatesBed OccupancyCervix MucusTime FactorsSupine PositionMuscular AtrophyMuscle, SkeletalPlasma VolumeEarly AmbulationExerciseUterine Cervical DiseasesHypotension, OrthostaticHypovolemiaVascular CapacitanceAmino Acids, EssentialBirth IntervalsLabor, ObstetricImmobilization